This invention is generally directed to liquid developer compositions and, more specifically, to liquid developers containing a bimodal molecular weight distribution ionic or zwitterionic ammonium block copolymers with a bimodal molecular weight distribution. More specifically, in embodiments the present invention relates to liquid developers with charge directors derived from alkylation or protonation of poly-2-ethylhexyl methacrylate-co-N',N'-dimethylamino-2-ethylmethacrylate (EHMA-DMAEMA) A-B diblock copolymers which form inverse micelles with the ammonium ionic or polar end of the block copolymer faced inward, and the nonpolar EHMA tail pointing in a direction outward toward the hydrophobic hydrocarbon vehicle selected for the liquid developer; and wherein the molecular weight distribution of the charge director is bimodal comprising a component with a number average molecular weight (determined by dividing the number of moles of monoinitiator into the number of grams of acrylic monomer being initiated by the charged molar quantity of monoinitiator) of from about 70,000 to about 200,000, preferably from about 80,000 to about 150,000, and more preferably about 85,000 to about 100,000, and a second component with a number average molecular weight of from about 2,200 to about 60,000, preferably from about 3,000 to about 20,000, and more preferably from about 4,000 to 10,000. Effective ratios of the high M.sub.n (number average molecular weight) over the low M.sub.n components ranges from 99/1 to 10/90 with a preferred range being 95/5 to 50/50. With the aforementioned molecular weight distribution, there are enabled liquid developers with a number of advantages such as high particle charge with a controlled range of low conductivities. The low conductivities result from the larger micelles which result from the high molecular weight component of the charge director. The large micelle reduces the conductivity in, for example as follows: 1) the electrophoretic mobility is reduced as the size of the micelle increases due to viscous drag; 2) as the size of the micelle increases, the number of micelles decreases at the same total mass loading of the charge director resulting in a decrease in the micelle charge density. Furthermore, it has been shown that these charge directors with selected molecular weight distributions result in low conductivity liquid toner dispersions with stable high particle charge over a range in low conductivities. For example, the charge director with the selected bimodal molecular weight distributions result in particle mobilities which vary from about 2.6 to 2.8 E-10 m.sup.2 /Vs over a conductivity (of 1 percent solids liquid toner dispersion) of from about 2 to 8 ps/centimeter. In contrast, the use of a low molecular weight monomodal molecular weight distribution charge director results in a mobility variation of about 0.2 to 2.5 E-10 m.sup.2 /Vs over the same conductivity range.
The developers of the present invention can be selected for a number of known imaging and printing systems, such as xerographic processes, wherein latent images are rendered visible with the liquid developer illustrated herein. The image quality, solid area coverage and resolution for developed images usually require sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system used. The electrophoretic mobility is primarily directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. A 10 to 30 percent change in fluid viscosity caused, for instance, by a 5.degree. C. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor image development and background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor transfer of the toner to paper or other final substrates. Poor or unacceptable transfer can result in, for example, poor solid area coverage if insufficient toner is transferred to the final substrate and can also lead to image defects such as smears and hollowed fine features. To overcome or minimize such problems, the liquid toners of the present invention were arrived at after substantial research efforts, and which toners result in, for example, sufficient particle charge for transfer and maintain the mobility within the desired range of the particular imaging system employed. Advantages associated with the present invention include a high developer particle charge over a range of low conductivities; and further increasing the desired negative charge on the developer particles and in embodiments providing a charge director that is superior to similar charge directors like tetraalkyl quaternary ammonium block copolymers, lecithin, and metal salts of petroleum fractions. Some of the aforementioned additives like lecithin contain impurities which can have an adverse effect on their intended function. The superior charge can result in improved image development and superior image transfer. The low conductivity of the dispersions obtained with the present invention improve the developability of the liquid toner dispersion as the high concentration of mobile ions in high conductivty liquid dispersions compete with the toner particles for the latent electrostatic image in the xerographic process. The high concentration of mobile ions, reduced with the present invention, can also disrupt other steps in the xerographic printing process such as the electrostatic transfer of the image from the image bearing member to a substrate. In a number of applications of the xerographic printing process, a subsequent electrostatic image is applied to the image bearing member over a previously developed image. In this process, often referred to as an image-on-image process, a high concentration of mobile ions in the first image would distort the electrostatic latent image being developed in the subsequent development. The desired low conductivity for any given electrostatic printing process will depend on specific features of the printing system and it is, therefore, desirable to have a liquid toner dispersion with an adjustable low conductivity which provides high particle charges over a range of low conductivities.
A latent electrostatic image can be developed with toner particles comprised of resin, pigment, and charge adjuvant dispersed in an insulating nonpolar liquid. The aforementioned dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, it is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet.
Useful liquid developers can comprise a thermoplastic resin, colorant like pigment or dye, and a dispersant nonpolar liquid. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 10 microns (.mu.m) average by area size as measured as determined by the Horiba Capa 500 or 700 particle sizers.
Since the formation of images depends, for example, on the difference of the charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and charge adjuvants which increase the magnitude of the charge, such as polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, and the like to the liquid developer comprising the thermoplastic resin, the nonpolar liquid and the colorant.
U.S. Pat. No. 5,019,477, the disclosure of which is totally incorporated herein by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors may include both negative charge directors, such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthanates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyl toluene and styrene, and (iv) selected from the group consisting of butadiene and acrylate.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles, may be selected from known thermoplastics, including fluoropolymers.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) consisting of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) consisting of a fluorine-free vinyl or olefin monomer block unit. The functional segment of block copolymer is oriented to the surface of the block polymer, and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner whereby the toner is provided with both liquid repelling and solvent soluble properties.
Moreover, in U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge additive. Liquid developers with charge directors are also illustrated in U.S. Pat. No. 5,045,425. Further, stain elimination in consecutive colored liquid toners is illustrated in U.S. Pat. No. 5,069,995. Additionally, of interest are U.S. Pat. Nos. 4,760,009 and 5,034,299.
The disclosures of each of the U.S. patents mentioned herein are totally incorporated herein by reference.
In copending U.S. patent application Ser. No. 986,316 the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., which developer has a melting point of at least about 25.degree. C., the contact occurring while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
In U.S. Pat. No. 5,306,591, there is disclosed a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and U.S. Pat. No. 5,308,731 discloses a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid.
In copending U.S. patent application Ser. No. 185,341, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director comprised of a zwitterionic quaternary ammonium block copolymer wherein both cationic and anionic sites contained therein are covalently bonded within the same polar repeat unit in the quaternary ammonium block copolymer.
In copending U.S. patent application Ser. No. 249,916, filed concurrently herewith and the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director comprised of an ionic or zwitterionic quaternary ammonium block copolymer ammonium block copolymer, and wherein the number average molecular weight thereof of said charge director is from about 70,000 to about 200,000.